Pitot tubes: unreliable and outmoded technology?

Iwannahurl

Given that pitot tubes were originally 18th/19th century technology, and that they have been possibly implicated in some serious plane crashes, can they be regarded as adequate for modern aviation purposes?

In the era of GPS and other technologies that can fly unmanned aircraft and missiles over huge distances with deadly accuracy, is measuring airspeed using the air pressure in a pipe really sufficient?

drdeadlift

This has crossed my mind too,its a pity it takes disasters for us to look to better our technologies

xflyer

Well, for the most part if you turn on the pitot heat, you won't go wrong. Keep it simple is what I say. Take away the pitot and someone jams the GPS and you have no airspeed readout?

Aviation is inherently conservative. For good reason. Reliability is most important. Take away the pitot and you must replace it with something as reliable and predictable.

Does GPS fulfil that requirement?

No it doesn't.

What airline pilots need is a bit more stick and rudder skills. So that when the instruments don't make sense they can revert to aviate, navigate, communicate.

Delta Kilo

To be honest the answes is no.

Firstly Pitot tubes are far more accurate than GPS, even with the new Galileo constellation of GPS satellites that are being rolled out, pitot tubes will always be more accurate.

Secondly, the most important info to the pilot re speed is his speed relative to the airflow, the air flowing over the wings, not his groundspeed, which GPS would provide. The pitot tube is essiantially a pipe, and the air flowing in puts pressure on another fluid. Depending on the pressure recorded from the fluid, the speed can accurately be calculated.

Yes it is an old technology, but it is brilliant in its simplicity. There have been issues with them freezing over so no air can get into the pipe, maybe a better idea would be to make the pitot heat automatic so pilots cant forget to turn them on.

Pitot tubes are used for other things aswell, they gather the dynamic and static pressure of the airstream for altitude purposes.

They are always going to be on planes as far as I can see.

Delta Kilo

PS - I love the way I did an exam on avionics 3 weeks ago and already I have forgotten half of it!

Iwannahurl

Mind you, having posted the above I Googled UAV and Pitot.

Looks like unmanned drones have them too, and that attempts have been made to devise algorithms that can help to increase fault tolerance by detecting any discrepancies between a UAV's airspeed as (a) measured by a pitot tube, (b) determined by GPS and (c) estimated from engine throttle.

So what is it about this 18th century invention that is still so essential?





EDIT: Overtaken by Delta Kilo's post above.

knockon

Interesting piece on Aviation Week.....


Although the pitot probes appear to have iced over, the speed discrepancy between the primary flight display and ISIS lasted around 45 sec., not atypical for the phenomenon.

There remain some long-term questions related to the AF447 accident. Would
better aural cues help focus pilot attention on recovering from a stall? One
industry official doubts replacing the "stall" alert with a more specific
instruction, such as "push stick," would make much of a difference. And
should research be funded toward devising a better backup mechanism to pitot
tubes or finding another way to determine true air space that is less
susceptible to outside environmental factors?

pclancy

I have often wondered why the ability to turn off pitot heat is even present? Why would you ever fly without it turned on? Should it not just energize once the aircraft has power or once its airbourne. Removing the chance of pilots forgetting to turn it on would make sense.

ArphaRima

I have often wondered why the ability to turn off pitot heat is even present? Why would you ever fly without it turned on? Should it not just energize once the aircraft has power or once its airbourne. Removing the chance of pilots forgetting to turn it on would make sense.

"Flight on batteries only" would last quite a bit less I would imagine in the event of multiple Bus or generator failures if it couldnt be shed. It should be automatic is what I think you are trying to say. Which it is in larger and more complex aircraft already.

The AF447 accident didnt happen because pitot heat was off. It is on always at high capacity whilst airborne.
Nor did the pilots entirely forget how to fly.
The stall warning was inhibited by airbus logic. That is what strikes me most about this accident.
When their speed tapes became unreliable, they could easily have disregarded the altitude tapes too. Stall buffet may have been pronounced, but not distinct from turbulent CB's thrashing them around.
Any pilot getting a STALL STALL STALL warning repeatedly for minutes will eventually react correctly no matter how slow-witted. They just werent getting that warning.

What airline pilots need is a bit more stick and rudder skills

How simple life must seem when you're a legend in your own mind.

Manic Moran

The gyroscopic attitude indicator is about a century old now as well, not much chance of that going to be replaced either.

Simple though it my be, nothing else is going to be as accurate. Airspeed at density altitude is hugely important a concept. It's the only thing which measures how many air molecules are going past the aircraft per unit of time.

Even if you hook up your RAIM GPS to the thermometer then have the computer crunch the numbers comparing groundspeed to throttle and attitude (Calculting for averages with changes in course to account for average windspeed and direction instead of simply groundspeed), you're not going to get as accurate an indication of what's going on right now.

NTM

pclancy

Fluffer I wasnt linking this thread with AF447 at all in my mind when I replied but anyways...yeah an automatic system would make sense.

fluffer wrote: »

How simple life must seem when you're a legend in your own mind.

I don't think he meant anything but what a lot of people think in and out of the industry and it wasnt an attempt at self promotion. I've sat at the club bar with pilots from both airline and charter sides of aviation that think the same. When you spend 95%+ of your actual pilot flying time without your hands on the stick or rudder how does that affect your actual flying ability?

Anyways I've dragged this more off topic when I was meaning to just speak about pitot tubes.

BrianD

Also you can't rely on GPS as it's owned by the US Government and is there for another purpose. While unlikely, service could be withdrawn or degraded without notice.

Ideally an aircraft should have an onboard and external means of calculating speed.

ArphaRima

Delta Kilo wrote: »

Secondly, the most important info to the pilot re speed is his speed relative to the airflow, the air flowing over the wings, not his groundspeed, which GPS would provide.
Yes it is an old technology, but it is brilliant in its simplicity.

Delta Kilo addresses this thread perfectly. Pitots are too useful and simple to abandon.

I also see need for changing and updating the use of alternative methods to determine speed. The best/quickest/cheapest way I see this happening is through software. ie Air Data Reference and Flight Control Logic.
Through increasing the credence given to GPS/ADIRU speeds they could provide more immediately accessible information to pilots in the event of airspeed discrepancies.

As for the flying ability comments; I take offense at the (preposterously)condescending tone towards (by definition) experienced professional pilots.
If I was to take this argument quickly to its final conclusion I would say the higher up the aircraft food chain the less crashes there are. This is due to a mix of experience, technology, types of airports frequented etc. The fact remains that the less a pilot has to handle the controls, the safer an aircraft and its passengers are. Statistically speaking of course.
1800ft on the way up and 1800ft on the way down is usual in direct control.
Now explain to me why an airline pilot should directly handle the primary flight controls more than 5% of the time. I think managing the flight management and guidance systems the other 95% of the time is good enough.

A320

Manic Moran wrote: »

The gyroscopic attitude indicator is about a century old now as well, not much chance of that going to be replaced either.


NTM

Done,on modern commercial aircraft Its called ISIS(Integrated Stby Instrument System)

pitot probes are automatically heated on modern commercial aircraft,when the aircraft lifts off they should switch on via an air/grnd relay logic or the LGCIU i/p on a Airbus,oh and just dont forget to pull the breakers when doing maintenance!!!!!

Manic Moran

A320 wrote: »

Done,on modern commercial aircraft Its called ISIS(Integrated Stby Instrument System)

You know, I utterly forgot about ring laser systems. A tad outside of my budget

NTM

tippilot

What some people have pointed out and a point some people are missing is that a GPS can only provide groundspeed information.

Groundspeed is useless information when it comes to keeping an aircraft in the air. Airspeed is what matters.

Consider an aircraft climbing. Your GPS may tell you that you have a groundspeed of 100kts, while your actual airspeed may be 150kts.

To put it another way: A light aircraft travelling at an airspeed of 70knots, while flying into a 70 knot headwind will have a groundspeed of 0 knots. In other words, as far as GPS is concerned it is going nowhere.

Now bring into that equation other localised variables such as headwinds, tailwinds, temperatures and pressures that GPS wont have any information about.

Now, now xflyer that surely is an unwarranted generalisation. Yes automatics play a major part in modern flight but the increasing use of automatics over the last few decades has a direct relationship with the higher standards of aviation safety we see and demand today.

The management of an aircraft's automatics is in itself a task that requires great attention. It does not place a pilot out of the loop. The opposite in fact. It allows a greater degree of oversight and the gives the excess mental capacity to see the bigger picture.

As for stick and rudder skills...you did have a good role model I suppose!!

Lustrum

xflyer wrote: »

What airline pilots need is a bit more stick and rudder skills. So that when the instruments don't make sense they can revert to aviate, navigate, communicate.

I can only presume this statement is to do with the AF crash - would you be suggesting that more "stick and rudder skills" are the answer to getting different readouts from your instruments at night at 36,000ft in the middle of a storm somewhere over the Atlantic?

To suggest that airline pilots don't have stick and rudder skills is ridiculous, granted they don't have to use them all the time, but all you have to do is read aviation herald to see the skill and capabilities of pilots landing safely in dangerous situations.

Delta Kilo wrote: »

Pitot tubes are used for other things aswell, they gather the dynamic and static pressure of the airstream for altitude purposes.

They are always going to be on planes as far as I can see.

Delta Kilo

PS - I love the way I did an exam on avionics 3 weeks ago and already I have forgotten half of it!

And you were doing so well! Pitot tubes are only used for airspeed readings, the other pressure instruments only use static pressure from the static port. I hope you passed the exam!!

enda1

Maybe hot wire anemometry or else laser-doppler anemometry could be used instead of/or as a redundancy back-up for air pressure based pitot measurements?

Delta Kilo

Lustrum wrote: »

And you were doing so well! Pitot tubes are only used for airspeed readings, the other pressure instruments only use static pressure from the static port. I hope you passed the exam!!

I may be wrong but I think I remember a pitot static tube being also used to aid with the measurement of another flight parameter other than airspeed... I may be wrong though, in fact, I probably am wrong. I would dig out the notes but they are the last thing in my mind now!

TW Mr Tayto

Delta Kilo wrote: »

I may be wrong but I think I remember a pitot static tube being also used to aid with the measurement of another flight parameter other than airspeed... I may be wrong though, in fact, I probably am wrong. I would dig out the notes but they are the last thing in my mind now!

Isn't it the pitot static SYSTEM?
The combination of the two could be used for many other things.

Ideas of when pitot could measure other things..
If ram air outlet is blocked, but not the pitot itself, it will only measure changes in atmospheric pressure - thus becoming a sort of altimeter (although a near useless one)

(That is coming without solid proof, it's an assumption.)


How embarrassing to make a mistake on my 100th post. Great milestone that would be!

Manic Moran

TW Mr Tayto wrote: »

Ideas of when pitot could measure other things..
If ram air outlet is blocked, but not the pitot itself, it will only measure changes in atmospheric pressure - thus becoming a sort of altimeter (although a near useless one)

(That is coming without solid proof, it's an assumption.)


How embarrassing to make a mistake on my 100th post. Great milestone that would be!

If both the pitot and the drain hole are blocked, then the airspeed indicator will perform as you state as it's still hooked into the static system on the other side of the diaphragm.

NTM

Shane Slv

would i be right in thinking that during a decent the gps speed would be lower then the actual airspeed. ?

Manic Moran

Assuming a zero-wind situation and a GPS which only calculated 2D speeds, yes.

NTM

triggermortis

Delta kilo - I'd start revising for a resit if I were you......

*Kol*

A320 wrote: »

pitot probes are automatically heated on modern commercial aircraft,when the aircraft lifts off they should switch on via an air/grnd relay logic or the LGCIU i/p on a Airbus,oh and just dont forget to pull the breakers when doing maintenance!!!!!

Not the case on the B737 classic or NG though. There will be an AD issued to modify the pitot heat system to automatically switch on when the engines are started (engine oil pressure will be the input signal). The switch on the overhead panel will be modified to have an auto and on position (instead of on and off). The logic behind this is that they do not want to change the after start checklist so pilots will still select the switch to on (even though the heaters will come on automatically when the engines are started). Research by the NTSB showed that some pilots were not selecting the pitot heat on whilst they were going through the after start checklist, and were waiting until they were at the threshold of the runway to switch on the heaters (this info came from DFDR downloads). Due to the high workload as they were just about to enter the active runway to take off, there were occasions where the pitot heaters were were forgotten. They do not know the reson for this practice but suspect that pilots were trying to save the pitot tubes from over heating and burning during long taxi's and during long queues to take off.

A320

Indeed you are correct i Should have mentioned the 737!! ha i did say modern!! Oh i can imagine the state of the paperwork already for that AD......... Boeings drawings are class